1. Technical Field
The present invention relates to internal combustion engines, particularly gas turbine engines associated with aircraft propulsion. More specifically, the invention relates to a method and apparatus for limiting ingestion of ground debris into the inlet of aircraft engines.
2. Background Art
Gas turbine engines for use with aircraft generally require large quantities of air flowing at high velocities to produce motive thrust or power generation. Turbojet and turbofan engines operated statically or for ground maneuvering tend to develop inlet vortices. Inlet vortices, which extend between the ground and a jet engine inlet, form around a stagnation streamline in the inlet flow provided there is sufficient ambient vorticity to continuously feed the vortex. A stagnation streamline forms whenever substantial amounts of air are being ingested by the inlet from behind and underneath the engine. The high rotational velocities of engine inlet vortices kick-up dust, dirt, and debris which may become undesirably entrained with the general inlet flow and ingested into the engine. Occasionally, objects are lifted by the low pressure region near the core of the vortices at the ground.
Devices have been designed to prevent or inhibit the formation of vortices between turbojet or turbofan inlets and the ground primarily during static operation or ground maneuvering when using forward thrust. Devices which have been used in service rely on engine compressor bleed air which is plumbed to the inlet lip to provide cowl ice protection and/or inlet vortex suppression. The earliest devices such as that shown in U.S. Pat. No. 2,915,262 issued to Klein, directed a jet of air onto the ground in front of the engine so that an inlet vortex could not reach the ground. Other devices such as shown in U.S. Pat. No. 4,070,827 issued to VanFleet et al, and U.S. Pat. No. 3,599,429 issued to Bigelis et al, direct jets of air below the inlet lip aft underneath the engine nacelle to prevent the formation of a stagnation stream line between the engine inlet and the ground by drawing/entraining air aft underneath the engine nacelle thereby blocking airflow underneath the engine from moving forward into the inlet. These devices are used on commercially operated, low wing transport aircraft having wing mounted engines hung close to the ground which are generally operated over relatively clean surfaces.
There is a class of high wing transport aircraft which has been developed to be capable of ground maneuvering on airfields having unimproved surfaces using high levels of reverse thrust to back up or to turn. In these types of aircraft, reverse thrust inlet vortex activity is generally observed to be worse than forward thrust vortex activity. Due to the large volumes of airflow entering each engine inlet during thrust reverser operation, and due to the increased height of the engines above the ground, conventional methods of discouraging inlet vortex formation using jets of air underneath the engine inlet would require relatively large amounts of highly compressed core engine gas path air. So much compressor bleed air would be required that the amount of compressor bleed air may also exceed the engine capability. Furthermore, large heavy ducting and special control systems would be required.